1. Field of the Invention
The present invention relates to an automatic focusing apparatus usable in an image pickup apparatus such as a digital camera.
2. Related Background Art
Heretofore, in an automatic focusing (AF) apparatus, a contrast detecting method, in which a contrast of a video signal of an object image is detected to execute focusing operation, has been widely used. In the contrast detecting method, a phenomenon that the contrast of the video signal increase as a focus state approaches an in-focus condition is used. In other words, while a focus lens is moved, a position (focus position) of the focus lens corresponding to a peak of a contrast value (AF evaluation value) is detected. A solid state image sensing element, such as CCD and CMOS, is used as a device for converting an optical signal of the object image into an electric video signal, and the contrast value can be acquired by extracting a high frequency component from the video signal. Meanwhile, the focus lens is driven when a lens holder with lenses fixed thereto is driven in a lens barrel by a driving source, such as a focus motor like a stepping motor, a linear motor or the like, through a mechanical system such as a nut, a screw and the like.
In an image pickup apparatus such as a digital camera, AF control, automatic exposure control, flashlight regulation control and the like are executed after a user pushes down a release button, and hence a release time lag occurs until photographing actually starts. Heretofore, techniques for reducing the release time lag have been developed to make it possible for a user to shoot without stress. Also with respect to the AF control, techniques for decreasing an AF time while improving an AF precision have been proposed. In Japanese Patent Laid-open No. Heisei-11 (1999)-155094, the following method is disclosed. In this method, when the AF evaluation value is acquired while the focus lens is moved, a driving velocity of the lens is calculated from a sampling frequency of the AF evaluation value and the total distance moving amount of the lens. And, the AF evaluation value is acquired while the lens is driven at the calculated velocity.
However, when the focus motor for driving the focus lens starts to be moved from a standstill condition, there is a possibility that the mechanical system cannot follow the motion of the focus motor due to adverse effects of inertia force and frictional force if the driving velocity is abruptly raised up to a certain velocity or more. Also in the stepping motor, if its drive is executed at a velocity above a certain velocity (a frequency of starting characteristics), a step-out phenomenon is likely to occur. Hence, the position of the focus lens is liable to be erroneously controlled.
Owing to characteristics of the motor, when acceleration operation is performed by gradually raising the velocity from a velocity below the velocity of the frequency of starting characteristics, a maximum velocity can be further increased. In the AF control, however, when the acceleration operation is performed, the displacement amount of the position of the focus lens varies if the AF evaluation value is acquired at a constant sampling period. Accordingly, it is likely that a position of the focus lens corresponding to a peak value of the AF evaluation value cannot be accurately calculated. Further, also when the lens is brought to a standstill, the step-out phenomenon is similarly likely to occur if the maximum velocity is abruptly brought to a standstill. Therefore, the velocity needs to be brought to a standstill through a deceleration operation without step-out phenomenon.